1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, in particular, to a bandgap reference circuit that is capable of having output voltages below the nominal bandgap value and of being operated from very low supply voltages with a simple, one temperature trim procedure.
2. Discussion of the Related Art
In prior implementations of low voltage bandgap reference circuits, a proportional-to-absolute-temperature (PTAT) current is added to a current that is proportional to a base-emitter voltage VBE such that a constant current is applied to a resistor, thereby creating a constant voltage. Some designs of this type include a buffer amplifier.
The major disadvantages of this design approach lie in the Early voltage error in the current sources and in the difficulty of implementing a precision buffer amplifier for very low supply voltages. Another disadvantage of this prior art is the difficulty of trimming the ratio of the PTAT and VBE currents in an integrated circuit production environment. Two temperatures are usually required to obtain a low temperature coefficient.
The present invention provides a bandgap circuit capable of having an output voltage below a nominal bandgap value (1.206V) and of being operated from very low supply voltages.
In a bandgap voltage reference circuit in accordance with the present invention, the different-sized emitters of the two bipolar devices of a xcex94VBE stage return to ground (or other bias voltage) through separate resistors. The VBE term of the reference device is supplied by a VBE current source through a third resistor. The proportional-to-absolute-temperature (PTAT) term of the reference occurs as the difference of base-emitter voltages xcex94VBE between the larger and smaller emitters. An output voltage Vout multiplier resistor feeds to the larger emitter through an inverting amplifier. In one embodiment of the invention, the output voltage Vout trim at one temperature is obtained by trimming the base-emitter resistor of the xe2x80x9csmall emitterxe2x80x9d device to compensate for the VBE process variation.